1. Field of the Invention
The present invention relates to a sheet processing apparatus which binds a sheet bundle with the sheet bundle folded into two, and to an image forming apparatus. More particularly, the present invention relates to a sheet processing apparatus capable of binding a sheet bundle in a good appearance so that unbound edges of sheets of the sheet bundle are unlikely to unfold.
2. Description of the Related Art
Some conventional image forming apparatuses, such as a copying machine and a laser beam printer, have a sheet processing apparatus that performs saddle stitch bookbinding by receiving sheets discharged after images are formed thereon. Subsequently, the sheet processing apparatus performs center-folding of a bundle of sheets or binding a substantially middle portion of the bundle of sheets and then folding the bundle of sheets into two.
Such a conventional sheet processing apparatus may have a bookbinding apparatus that saves space, that is small, and that is low-priced. In such a bookbinding apparatus, in a case where a sheet bundle is bound, a predetermined number of sheets discharged one by one are carried into a substantially vertically longitudinal accommodation guide.
Then, leading edges in a sheet carrying-in direction of the sheets are positioned by being made to contact a sheet positioning member preliminarily placed at a predetermined binding position. Subsequently, alignment in a width direction of the sheets is performed. Then, central portions of the sheets are saddle-stitched by a stapler so as to form a sheet bundle. Next, the saddle-stitched portion of the bundle of the sheets is pushed between a folding roller pair. Thus, bookbinding is performed by folding the middle portion of the sheet bundle. Then, the bound sheet bundle is discharged, with the fold position in the lead, to a discharge tray from a discharge port provided at a discharge side of the folding roller pair (see Japanese Patent Application Laid-Open No. 11-193175, corresponding to U.S. Pat. No. 6,276,677).
FIG. 15 illustrates a conventional image forming apparatus 300 and a conventional sheet processing apparatus 501, which has a conventional bookbinding apparatus. In a case where a sheet bundle is bound, the sheet processing apparatus 501 first receives a sheet S discharged from the image forming apparatus 300. Subsequently, the sheet processing apparatus 501 carries the sheet S into a saddle stitch processing tray 540. Thereafter, the sheet processing apparatus 501 performs alignment of the sheet S. This process is performed until a predetermined number of sheets S are carried into the saddle stitching tray 540.
Next, when a predetermined number of sheets S are conveyed to the saddle stitch processing tray 540, stitching (or two-position binding) of a sheet bundle is performed to bind the sheet bundle. Subsequently, a sheet positioning plate 71 supporting the sheet bundle from below is lowered to a position at which the height of the center of the sheet bundle is equal to that of a sheet pushing plate 72 and that of a nip portion between sheet folding rollers 73.
Then, the center of the sheet bundle is pushed by the sheet pushing plate 72. Thus, the center of the sheet bundle is moved towards the sheet folding rollers 73. Consequently, the sheet bundle is nipped by the sheet folding rollers 73 to be folded into two. The sheet bundle folded into two is discharged, with the fold position in the lead, to a saddle stitch discharge tray 515 by the sheet folding rollers 73 and a discharge roller pair 74.
However, the conventional bookbinding apparatus performing such a conventional bookbinding process and the conventional image forming apparatus have drawbacks in that as the number of sheets of a sheet bundle increases, stiffness of the sheet bundle increases, thus resulting in insufficient folding, so that unbound edges of sheets of the sheet bundle are likely to unfold, and that the appearance of the bound sheet bundle is not good. Also, the conventional apparatuses have another drawback in that because unbound edges of sheets of the sheet bundle are likely to unfold, it is difficult to stack a plurality of sheet bundles.
To solve the above drawbacks, an apparatus has been developed, which forms a groove on a fold line portion of each sheet of a sheet bundle to facilitate sufficiently folding the sheet bundle (see Japanese Patent Application Laid-Open No. 2000-272823). However, the groove formed on each sheet of the sheet bundle has a uniform width. Accordingly, the conventional apparatus has drawbacks that in a case where the thickness of the sheet bundle increases, the outer the sheet of the sheet bundle, the more difficult the folding of the sheet is, and that the appearance of the sheet bundle is worsened after the sheet bundle is folded.